1. Field of the Invention
This invention relates to an objective lens, and particularly to an objective lens suitable for use in a color video camera having a prism block having dichroic film on the image side of the objective lens and dividing a light beam into a plurality of color components.
2. Related Background Art
In a three-tube type TV camera, a three-color resolving optical system comprising a plurality of prisms, color filters or the like is provided as means for color-resolving an incident light beam into a plurality of predetermined color lights between a photo-taking lens and an image pickup plate (or an image pickup tube or a solid state image pickup element).
This three-color resolving optical system is composed chiefly of a prism block comprising three to four prisms of a predetermined shape each having multi-layer film, for color resolution provided on one surface thereof and combined as a unit while having a predetermined relation therebetween. The optical system is designed to resolve a light beam incident from the photo-taking lens into color lights of plural wavelength bands through the reflecting surfaces for color resolution of the prisms and direct the color lights to respective image pickup plates.
FIG. 9 of the accompanying drawings is a schematic view of a prior-art three-color resolving optical system comprising three prisms. In FIG. 9, the reference numeral 91 designates a photo-taking lens, the reference numeral 92 denotes a blue resolving prism, and the reference numeral 93 designates a red resolving prism having its entrance surface 93b disposed with a minute gap with respect to the blue resolving prism 92. The reference numeral 94 denotes a green resolving prism having its entrance surface 94a adhesively secured to the reflecting surface 93a of the red resolving prism 93.
In FIG. 9, of a light beam passed through the photo-taking lens 91, blue light B is separated by the light dividing surface 92a of the blue resolving prism 92, which is coated with dichroic film, and is totally reflected by the entrance surface 92b of the blue resolving prism 92, whereafter it emerges from the exit surface 92c of the blue resolving prism. Red color R is separated by the light dividing surface 93a of the red resolving prism 93, which is coated with dichroic film, and is totally reflected by the entrance surface 93b of the red resolving prism, whereafter it emerges from the exit surface 93c of the red resolving prism. The remaining green light G, than the blue and red components passes through the green resolving prism 94 and emerges from the exit surface 94c of this prism. The blue light B, the red light R and the green light G, thus, total three light beams, and are directed to image pickup plates, not shown.
In the prior-art three-color resolving optical system shown in FIG. 9, when light beams U (dot-and-dash line), C (solid line) and L (dotted line) passes through the central portion and marginal portion of the photo-taking lens as shown enter the blue dichroic film provided on the light dividing surface 92a of the blue resolving prism 92 and the red dichroic film provided on the light dividing surface 93a of the red resolving prism 93, which prisms are positioned in the course of the arrival of the light beams at the respective image pickup plates, not shown, there are created differences among the angles of incidence of the light beams U, C and L. Therefore, the spectral transmission characteristics for the light beams U, C and L become different from one another as shown, for example, in FIG. 10 of the accompanying drawings. Respective lines in FIG. 10 correspond to respective lines in FIG. 9. In FIG. 10, the abscissa represents wavelength.
Therefore, when the color reproduction of a color image is effected in an in-focus state (a position Y indicated in FIG. 9), no color misregistration occurs to the object image, but when the color reproduction of a color image is effected, for example, in a state in which the object is deep and is out of focus (positions x and z indicated in FIG. 9), a blurred image gradually becomes colored in the vertical direction thereof.
Assuming that the image pickup surface lies at the position Y, when the coloring of the blurred image at this time is watched on a color monitor, the blurred image on the image pickup surface Y, when the position x is the image point, will appear as magenta in the upper portion thereof and as green in the lower portion thereof, as shown in FIG. 11 of the accompanying drawings. Also, the blurred image on the image pickup surface Y when the position Z is the image point will conversely appear as green in the upper portion thereof and as magenta in the lower portion thereof, as shown in FIG. 12 of the accompanying drawings. Thus, there has been problems in that the quality of the image, when a color image which is out of focus is represented, becomes greatly deteriorated.